Support for dnslink

[agazso]

Currently Bee (and the gateway-proxy) only supports ENS domains. The issue with those are that it's quite expensive to buy them but more importantly it's also quite expensive to update or modify them because of mainnet gas prices.

One idea is to support dnslink on the gateway-proxy, at least to some degree. For example if the proxy served unknown A records by looking up their dnslink TXT record then it classic DNS domains could still work but the content could be served through the proxy.

[agazso]

The specification of the configuration as per our meeting is the following:

There is a configuration variable called DNSLINK which is a boolean and defaults to false. When this is enabled then there is another configuration variable called DNSLINK_ALLOWED_DOMAINS which defaults to undefined or it can be a comma separated string and it contains a list of allowed domains. When there are no domains specified it tries to serve any kind of domain that it receives in the Host header of the HTTP request. Otherwise the gateway-proxy would first check if the Host header in the request matches and of the domains in the list (if not then 403) and serve that.

The feature works as a complementary feature to the CID_SUBDOMAIN and ENS_SUBDOMAIN features.

[luissanchez0305]

@agazso 1 question: If the Request is going to response 403 status, then that's the end of the request.
How would DNSLINK feature complement the CID and ENS if the request had ended at that point?